Compared with conventional rotationally symmetric surfaces, freeform surfaces have asymmetric surfaces and more degrees of design freedom, which can reduce the aberrations and simplify the structure of the system. In recent years, freeform surfaces are often used in off-axial three-mirror optical system.
However, conventional off-axial three-mirror optical systems with freeform surfaces are mostly applied to a small field of view, and the F-number of the system is large. In addition, a primary mirror, a secondary mirror, and a tertiary mirror in the off-axial three-mirror system are all freeform surfaces, and the primary mirror and the tertiary mirror are separate from each other; which makes the off-axial three-mirror optical systems are difficult to be assembled and fabricated.